Which events may cause long-term changes in climate?

In our ecosystem or current environment, The most scary reason that might happen is the volcanic eruptions. It will cause a long term changes in the climate since when it erupted, together with the lava are many chemicals that mixes with the air.

Droughts cause very arid places such as a desert

There are  advantages  to being  multicellular  rather than unicellular. these include allowing: the  organism  to be larger.